This is an example of how to do oauth in Java, via freelancer.com's api. 
No libraries are used outside of apache codec for encryption.
To get started you'll need to create a consumer key at freelancer.com and enter the token and secret they provide 
into applicationContext.xml. After that just do a mvn package and deploy the war. 
Then bring up index.html which has 2 example link going through the proxy (which will trigger the oauth handshake)
From there I suggest stepping through the java. The only part that is confusing is bouncing 
around between my various servlets and freelancer. This is just how the oauth handshake works, 
read some stuff on oauth first and this will become much clearer. 
I wish I had some nice code like this to follow along with when trying to learn and write my own, so here it is!
Also please understand this is just a quick example I used to learn, so code harsh code critics ;)
If you have a compliant send me a pull request, I promise to take almost anything.